Magnetoelectric multiferroics—materials that simultaneously show some form of magnetic and ferroelectric order—have excited condensed-matter researchers worldwide with the promise of coupling between magnetic and electric order parameters. A Berkeley–Stanford–Swiss group has now used the multiferroic bismuth–iron–oxygen compound BiFeO3 (BFO) to explore electrical control of magnetism through exchange coupling with a ferromagnet. Their experiments reveal the possibility of controlling ferromagnetism with an electric field at room temperature, a capability that could result in new and novel devices for magnetic data storage, spintronics, and high-frequency magnetic devices.

Left: Schematic illustrating how the connection between ferroelectricity, antiferromagnetism, and ferromagnetism results in electrical control of magnetism. Right: Exchange interaction between the multiferroic BiFeO3 and the ferromagnetic Co0.9Fe0.1, the right leg of the triangle, results in an enhanced coercive force (magnetic field required to reverse the magnetization) for the ferromagnet (red) relative to that of the ferromagnet grown without the BiFeO3 (black).

Electrical Control of Magnetism

Among the many materials with the magnetic and electric properties essential to our modern technological society, permanent (or ferro) magnets have a spontaneous magnetization giving rise to an intrinsic magnetic field. Similarly, a ferroelectric has a spontaneous electric polarization giving rise to higher voltage on one side than on the other. Ferroelectrics are used in diverse applications from barbeque lighters to motion sensors and non-volatile memory for data storage. Unusual materials called multiferroics simultaneously possess some form of magnetism and ferroelectricity.

Recently, promising new classes of multiferroics have been discovered. In some of these, magnetism and ferroelectricity are strongly coupled, raising the prospect of using one property to control the other. Not only is this coupling of fundamental interest to solid-state physicists, it is also exciting to those in the technology word for it raises the possibility of electrical control of magnetism in innovative new devices for information storage and in electronics (so-called spintronics). Martin et al. have demonstrated this possibility in a sandwich comprising the mutliferroic bismuth–iron–oxygen compound in intimate contact with a ferromagnet. By exploiting both the magnetoelectric coupling within the multiferroic compound and the magnetic coupling between the multiferroic and a ferromagnet, they were able to control the magnetism in the ferromagnet by applying a voltage to the multiferroic, an essential step that could lead to dramatic shift in memory design and functionality.

BFO is an antiferromagnetic, ferroelectric multiferroic with a Néel temperature of about 370ºC and a Curie temperature near 820ºC, respectively. Like a ferromagnet, which has a spontaneous magnetic dipole moment, a ferroelectric has a spontaneous electric dipole moment or electric polarization. The researchers developed a simple heterostructure device on a strontium titanate substrate to achieve deterministic control of ferromagnetism in a film of the cobalt–iron compound Co0.9Fe0.1 with an in-plane electric field applied to an underlying BFO layer. Growth of the Co0.9Fe0.1 film in a magnetic field aligned the initial ferromagnetic domains with the field, establishing a reference orientation (uniaxial anisotropy). Then, field pulses applied via in-plane strontium ruthenate electrodes reoriented (switched) the ferroelectric domains in the BFO.

The experiment made use of two types of electromagnetic coupling phenomena that are manifested in this type of heterostructure. The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BFO film that couples the film’s ferroelectric to its antiferromagnetic order. The second is based on exchange interactions at the interface between the two materials that couples the antiferromagnetic order of the BFO to the ferromagnetic order of the Co0.9Fe0.1. The researchers probed the inter-dependent coupling between these orders with piezo-response force microscopy (PFM) and photoemission electron microscopy (PEEM) at ALS Beamline 7.3.1.

In-plane PFM images of the BFO layer for this device structure in the as-grown state and after the application of electrical field pulses revealed the presence of a set of two stripe-like ferroelectric domains running at 45° to the in-plane contacts that switch by 90° back-and-forth in a repeatable fashion. Analysis of the intensity distribution in the PEEM images of the Co0.9Fe0.1—based on x-ray circular dichroism contrast at the cobalt L-edge—in the same sequence enabled the group to determine the direction of its local magnetization. The observations strongly indicated that the average magnetization direction in the ferromagnet rotated by 90° upon the application of the electric field to the BFO. Upon switching the BFO once again, the average magnetization direction changed back to the original state.

Left: 3D and cross-sectional schematic diagrams of the coplanar epitaxial electrode device showing the structure that enabled controlled ferroelectric switching and, hence, electrical control of local ferromagnetism in the Co0.9Fe0.1. The structure is grown on a strontium titanate substrate and has strontium ruthenate electodes. Right: In-plane PFM images showing the set of two stripe-like ferroelectric domains (black and brown contrast) running at 45º to the in-plane contacts. The domains switch by 90º for a device relative to the as-grown state following electrical switching and back again after the second switching.

Comparing the two techniques revealed strong correlation between the ferroelectric domains in the BFO and ferromagnetic domains in the Co0.9Fe0.1, mediated by the collinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. In short, the matching of the domains suggests the ability to control ferromagnetism with an applied electric field. This represents a significant advance in the field of multiferroics, as it marks the first demonstration of a possible room-temperature application to utilize multiferroic materials in a novel new device.

The authors caution that these promising results notwithstanding, the observations are still early in our understanding of the details of the coupling in such heterostructures. Furthermore, the exact details of factors such as the BFO surface roughness, shape and thickness of the Co0.9Fe0.1 layer, the magnitude of the applied magnetic field during the growth process, and most important, the magnitude of the antiferromatic-ferromagnetic coupling energy relative to other energy scales in this coupled system (magnetostatic energy and magnetocrystalline anisotropy energy) need to be carefully examined in future experimental and theoretical studies.

Top: PEEM images of the ferromagnetic domain structure (black, gray, and white) of the Co0.9Fe0.1taken in the as-grown state, after the first electrical switch, and after the second electrical switch. Bottom: Schematic diagrams of the observed magnetic contrast in the PEEM images, where the black, gray, and white arrows indicate the magnetization directions of the domains. The images reveal that the net magnetization of the Co0.9Fe0.1 (orange arrow) rotates by 90º upon application of an electric field.